The present invention relates to a cooling arrangement for blades of a gas turbine or the like.
It is generally known for blades of a gas turbine which are subjected to high thermal loading to be provided with a cooling arrangement. In the thermal combined-cycle power plants primarily relevant here, in order to cool the components subjected to thermal loading air is often extracted from the process gas stream, with the result that the overall efficiency of the plant is sometimes seriously impaired.
According to EP 0 674 009 A1, therefore, it was proposed, for cooling purposes, to draw saturated steam from a waste-heat steam generator or superheated steam from the steam circuit and supply it to the components to be cooled. The steam is subsequently led back into a steam turbine of the steam circuit at a suitable point. A principle advantage of this concept is that an improvement in the cooling action can be achieved on the account of the specific heat capacity of steam, so that a design based on a higher hot-gas temperature is possible. Furthermore, insofar as cooling is carried out in a closed circuit, improved efficiency of the combined-cycle plant is obtained.
In this respect, the configuration of the cooling ducts of the blades to be cooled assumes particular importance, since these are critical for the utilization of the cooling potential of the steam and the equalization of the component temperature. DE 19860787.3, from which the invention proceeds, discloses an optimized cooling arrangement for blades of a gas turbine, the blades being built up in each case from a suction-side and a pressure-side wall which are connected to one another, to form a cavity, via a leading edge, a trailing edge, a blade tip and a blade root. Integrated in the cavity is a flow path in the form of a multiplicity of cooling ducts, through which a cooling medium, in particular steam, is capable of flowing. Each blade of a corresponding blade row possesses a supply duct, via which the steam is fed in, and an outflow duct, via which the steam leaves the respective blade again.
Although cooling arrangements of this type have proved most appropriate, in many cases they cannot yet be considered optimal from the point of view of the utilization of the cooling potential of the steam led through them. The outlay in terms of construction is also considerable, since both a supply duct and an outflow duct for the steam must be assigned to each blade. Finally, appreciable flow losses occur when the steam is led by the blade, since the deflection of the steam in the region of the blade root has to take place in an extremely restricted space.
The invention attempts to avoid the disadvantages described. The object on which it is based is to specify a cooling arrangement for blades of a gas turbine of the type initially mentioned, which allows an improved utilization of the cooling potential of the steam led through and, furthermore, makes a simplified construction possible.
This is achieved, according to the invention, in that in each case two or more adjacent blades are combined and the flow paths are configured as a continuous cooling duct sealed off relative to the hot-gas stream. A greater quantity of heat can thereby be supplied to the steam flowing through, thus increasing the efficiency of the steam cycle. Moreover, the number of connections required is reduced, since only one supply duct and one outflow duct have to be provided for each blade group. The number of connections required is halved simply by two blades being connected to form a twin blade. When multiple blades are concerned, this effect can be increased even further, since the blades in each case located on the inside do not need connections of this type.
Expediently, the cooling medium is first supplied to a distributor space and is introduced from there into the cooling ducts of the blades assigned to this distributor space. A corresponding collecting space, out of which the heated steam is discharged, is provided on the outflow side.
The distributor space and collecting space may be built up particularly cost-effectively from tubes in the form of segments of an arc of a circle. These tubes make it possible to have in each case a congruent design of the distributor space and collecting space, these being mounted mirror-symmetrically to one another. The diversity of components can be considerably reduced in this way.
Preferably, the distributor space and the collecting space are mounted in a casing portion. An extremely space-saving and axially short-sized turbine stage can be produced as a result.
Corresponding to this, a deflecting space may be integrated as a crossover from one blade to the adjacent blade in the region of a platform portion, cooling of the blade root region and hub region thereby additionally taking place.
A further variant also tends in this direction, in which the steam at the outlet of the respective blade is used even further for cooling a heat accumulation segment which surrounds an adjacent moving blade row. In this case, the cooling duct is thus prolonged beyond the region of the blade row primarily to be cooled and consequently allows the efficient cooling of a complete turbine stage.
Depending on the construction of the blade to be cooled, the cooling duct may be built up from a plurality of part ducts running essentially parallel. This measure allows optimum distribution of the steam and directional adjustment to those regions of the wall which are subjected to particularly high thermal stress.
A variant provides for individual part ducts or groups of part ducts to be arranged separately from one another in a fluidtight manner. This design ensures that there is no intermixing of the individual part cooling streams. This effect may advantageously be utilized, for example, either to supply different cooling media, or to use cooling media having different state variables directionally to specific regions of the blade, in order thereby to effect optimum adjustment to temperature distributions imparted from outside.
A series of further preferred embodiments is also to be seen from this aspect, intended, in particular, for compensating radial temperature gradients and, furthermore, allowing for the fact that heating takes place when the steam passes through the flow duct, with the result that the temperature difference, available for heat transmission, between the steam and the blade wall changes.
In order to set a constant heat discharge or heat transmission condition, on the one hand, there may be provision for the cooling duct to have a cross-sectional profile decreasing in the direction of flow of the cooling medium. For most applications relevant in practice, it is not necessary to implement an idealized continuous cross-sectional reduction. It is often sufficient, instead, to integrate a geometrically simply constructed displacement body in the flow path at a suitable point. A particularly simple construction is obtained when the displacement body is arranged continuously between the blade root and blade tip of a blade. In the case of a twin blade, for example, the first blade is provided with a cavity running rectilinearly, whereas the second blade, whilst having an identical cavity contour, is provided with a displacement body inserted into the cavity. The displacement body may have a variable cross section, for example a cross-sectional profile increasing in the direction of flow, so that the residual cross section remaining in the cavity is largely approximated to the ideal calculated cross-sectional profile.
Alternatively, it is also possible to leave the cross section unchanged in the direction of flow and, instead, provide turbulence-generating elements, for example in the form of baffle plates or webs. Furthermore, the area of these elements may increase in the direction of flow, thus taking into account the increase in the coolant temperature by an increase in the coefficient of heat transmission. Mounting is recommended, in particular, in portions where there is high thermal loading, that is to say, in particular, in the region of the leading edge.